Michael J Fox expresses his support for stem cell research, and the results are almost unwatchable. His gaze remains fixed on the camera - his body, however, is lost in successive waves of lurching, jolting tremors. So goes the TV ad the actor - long diagnosed with Parkinson's disease - has released in support of Claire McCaskill, Missouri's Democrat candidate for the US Senate, inspired by her advocacy of stem cell research as a means of finding a cure for Parkinson's.

Almost unwatchable - but, it seems, only almost. Recently posted on YouTube, the spot has now pulled in over 1.4 million views (almost certainly more by the time you read this), its global reach some way beyond that usually achieved by the campaign materials of Missouri's race for the senate.

Much of that interest has, you imagine, arisen from the malodorous controversy that blew up almost instantly around the ad. Barely had it aired than conservative commentator and professional moron Rush Limbaugh waddled into action with the claim, apparently informed by the twin counsel of Alan Partridge and Satan, that Fox was exaggerating his symptoms. "He's moving all around and shaking and it's purely an act," Limbaugh observed, before opining: "This is really shameless of Michael J Fox. Either he didn't take his medication, or he's acting."

(Was there, perhaps, a glimmer of envy in Limbaugh's reference to medication? The former baton-twirler for the US War on Drugs has presumably had to foreswear his own after being exposed as addicted to pain-killers back in 2003.)

But even as the world recoiled, the Republicans were unveiling a marginally less noxious response - a rival ad denouncing stem cell research, employing their own celebrity star power. To wit, the spot features Patricia Heaton of TV's Everyone Loves Raymond, and the Hollywood actor Jim Caviezel. The latter's bizarre introduction is still puzzling viewers who appear unsure as to whether it was delivered in Aramaic (and thus designed to milk his turn as Jesus in Mel Gibson's Passion of the Christ), or simply, to quote the New York Times, "garbled." Maybe Rush Limbaugh was in charge of on-set refreshments?

The sad irony about the original ad is that, if anything, it's clear Fox is trying to restrain the effects of his condition in order to keep his speech coherent and his message understood. Yet, for all its discomfort, people just keep tuning in. Inevitably, you can't help but wonder what proportion of those myriad YouTube views have been down to news values, and how much are a result of the morbid curiosity that routinely surrounds Hollywood's genuinely tragic declines. It was this same rubbernecking that dogged the later lives of Christopher Reeve and Richard Pryor.

No one here is judging - I'm as likely as the next gawper to be found with the National Enquirer in the aisles of Tesco, grubbily poring over a story headlined: "Mr T Looks Worse Than Ever!" But it's still sobering to realise how many of us remain so drawn to a real-life narrative that precisely inverts that of Dorian Gray, the cruel phenomenon of portraits committed to celluloid remaining forever vibrant and pristine, while the subjects fall prey to the most merciless public ravages. Still, at least we know when we're going to hell - eh, Rush?